1. Technical Field
The present invention pertains to methods and apparatus for producing sterile slush for use in surgical procedures and for maintaining sterile fluid at a preselected constant temperature. The invention is an improvement of the methods and apparatus disclosed in U.S. Pat. Nos. 4,393,659 (Keyes et al), 4,934,152 (Templeton) and 5,163,299 (Faries, Jr. et al). The entire disclosures of those patents are expressly incorporated herein.
The invention is also an improvement of the methods and apparatus disclosed in the following commonly owned patent applications: U.S. patent application Ser. No. 08/125,279, filed Sep. 23, 1993, now U.S. Pat. No. 5,331,820, by Durward I. Faries, Jr., Bruce Heymann and Mark Licata, and entitled "Method and Apparatus for Producing Surgical Slush and Heated Sterile Liquid" and U.S. patent application Ser. No. 08/033,639, filed Mar. 16, 1993, now U.S. Pat. No. 5,333,326, by Durward I. Faries, Jr. and Bruce Heymann and entitled "Method and Apparatus for Producing Sterile Slush". The entire disclosures of those patent applications are also expressly incorporated herein.
2. Discussion of the Prior Art
The above-referenced Keyes et al patent discloses a surgical slush producing system having a cabinet with a heat transfer basin at its top surface. A refrigeration mechanism in the cabinet takes the form of a closed refrigeration loop including: an evaporator in heat exchange relation to the outside surface of the heat transfer basin; a compressor; a condenser, and a refrigeration expansion control. A separate product basin is configured to be removably received in the heat transfer basin. Spacers, in the form of short cylindrical stubs or buttons, are arranged in three groups spaced about the heat transfer basin and projecting into the heat transfer basin interior to maintain a prescribed space between the two basins. During use, that space contains a thermal transfer liquid, such as alcohol or glycol, serving as a thermal transfer medium between the two basins. A sterile drape, impervious to the thermal transfer medium, is disposed between the product basin exterior and the liquid thermal transfer medium to preserve the sterile nature of the product basin. Surgically sterile liquid, such as sodium chloride solution, is placed in the product basin and congeals on the side of that basin when the refrigeration unit is activated. The refrigeration unit is arranged to refrigerate the heat transfer basin to a temperature below the freezing temperature of the liquid used to form the slush. That temperature is described in the patent as being on the order of 25.degree. F. to 28.degree. F. A scraping tool is utilized to remove congealed sterile material from the product basin side to thereby form a slush of desired consistency within the product basin.
The Keyes et al patent describes the sterile fluid as being suitable for subcutaneous administration and preferably approximately isotonic, an example of which is a normal (0.85-0.9%) sodium chloride solution. The refrigerant expansion control is described as possibly being of the type that operates in response to the temperature of the heat exchange basin to control the flow of refrigerant to the evaporator in a manner to maintain the temperature at the heat exchange basin at a preselected value below the freezing temperature of the sterile liquid.
As noted in the above-referenced Templeton patent, the Keyes et al system has a number of disadvantages. In particular, the separate product basin must be removed and re-sterilized after each use. Additionally, the glycol or other thermal transfer medium is highly flammable or toxic and, in any event, complicates the procedure. The Templeton patent discloses a solution to these problems by means of an apparatus wherein the product basin is eliminated in favor of a sterile drape impervious to the sterile surgical liquid, the drape being made to conform to the basin and directly receive the sterile liquid. Congealed liquid is scraped or chipped from the sides of the conformed drape receptacle to form the desired slush.
The system disclosed in the Templeton patent has two operating modes. In one mode the basin is cooled to a temperature below the freezing point of the sterile liquid. In the other mode the contents of the basin are heated to a temperature slightly elevated with respect to normal human body temperature, typically on the order of 105.degree. F.
A problem with both the Keyes et al and Templeton systems is that, with respect to providing surgical slush, they are either on or off. When the system is on, the basin is cooled to below the freezing temperature of the sterile medium to form the slush. If the systems are kept in the on mode too long, the medium freezes further to form solid chunks of ice. When the systems are turned off, the sterile medium melts. What is needed for surgical use, however, is continuous control over the sterile fluid temperature, particularly the semi-frozen slush phase of the medium, but also including providing a supply of chilled liquid-phase medium.
It is also noted that, in the cool or on mode of the Keyes et al system, the heat exchange basin is cooled to 25.degree. F. to 28.degree. F., i.e., only a few degrees below the freezing point of the sterile medium. This has the disadvantage of taking an inconveniently long time to cool the basin and the sterile medium to the desired temperature. The Templeton patent is silent as to the temperature to which the basin is cooled.